I have a customer where my company installed a 404 compressor less than a year ago on a walk in freezer. The guy who installed it no longer works for me. I know he installed the right compressor because the old compressor is still sitting outside this restraunt and its an exact match. Ok well I got a call from the owner yesterday says freezer not getting cold enough...well I showed up not knowing what refer was in the unit because there is no listing on the condensing unit. I looked in the evap and it had a r12 txv in it...so i evacuate and put in mp66 which is a drop in r12...temp reallly never comes down that good. Well after looking in my computer at this job i find out 404a was put in by my prior employee so this thing has ran all night with mp66 in it....my question is how will this effect my compressor. Im going back today to replace the expansion valve to a 404a expansion valve and put 404 back in this system but have I ruined this compressor by letting r12 run in it over night? My past employee undoubtedly just changed this compressor with what he matched with that was already there unknowing that the expansion valve wasnt right.